list=[m+n for m in 'ABC' for n in 'ASD']
print(list)
list=[m for m in range(1,20) if m%5==0]
print(list)
import os
list=[d for d in os.listdir()]
print(list)
#课后习题：有些不是字符串，不能使用lower()方法，试着找出解决办法
L1=L1 = ['Hello', 'World', 18, 'Apple', None]
L2=[s.lower() for s in L1 if isinstance(s,str)==True]
# 测试:
print(L2)
if L2 == ['hello', 'world', 'apple']:
    print('测试通过!')
else:
    print('测试失败!')